An inkjet recording method has become rapidly popular in these days because it can record color images on plain paper and its running cost is low. However, the inkjet recording method has problems that image defects typified by character feathering (hereinafter, referred to as feathering) easily occur depending on the combination of an ink with a recording medium used, causing significant degradation in image quality.
Therefore, an attempt has been made to prevent feathering by suppressing the permeability of the ink. In this case, however, the drying properties of the ink degrade, which cause problems that the ink stains a hand as a resulting print is touched by the hand, or image smear occurs.
When a color image is recorded by the inkjet recording method, different color inks are sequentially superimposed one another. As a result, color inks bleed and are mixed together at the boundary parts of colors (hereinafter, referred to as color bleeding), and image quality significantly degrades.
To solve these problems, attempts have been made to prevent the color bleeding by increasing ink permeability. However, in this case, since a colorant penetrates inside the recording medium, image density degrades, and ink significantly strikes through to the back surface of the recording medium, failing to perform double face printing in a suitable manner.
To solve these problems and improve image quality, image forming methods using a treatment liquid and an ink have been proposed.
These proposed methods include, for example, a method for forming a coloring portion on a recording medium using a liquid composition in which particles, each surface of which is charged with the reverse polarity to that of an aqueous ink, are dispersed (see PTL 1), a method for performing printing by depositing an ink composition and a first liquid containing polymer particles on a recording medium for improving abrasion resistance of a resulting image formed product (see PTL 2), a method for improving image density and smear fixing ability of resulting images using a combination of a cationic polymer compound and organic acid (see PTL 3), a method for improving image density of resulting images by applying a highly viscous treatment liquid containing a cationic polymer compound (see PTL 4), and a method for improving a fixing ability of recorded images by specifying the viscosity of a treatment liquid containing amphoteric polymer compound, and the viscosity of a recording liquid containing a colorant to certain ranges (see PTL 5).
A roller coating device is used for an application of a treatment liquid. Since the roller coating device has a configuration having a combination of larger numbers of rollers, it is however difficult to seal the device completely. As a result, in the case where a device is not used over a long period of time, the treatment liquid may be deposited and solidified on a roller or in a tank, or may become viscous, which makes the application of the treatment liquid difficult.
As a measure for solving this problem, there are a method for sealing a tank with a rubber member (see PTL 6), and a method for increasing an amount of a moisturizing agent in an treatment liquid (see PTL 7). Even with the method for sealing the tank with the rubber member, however, it is impossible to completely prevent drying of the treatment liquid as these are many operation units in the device. Moreover, the method for increasing the amount of the moisturizing agent in the treatment liquid has side effects that the viscosity thereof increases, or an effect of improving the image density is weaken.
Considering the aforementioned problems, it is possible to provide an ink having low viscosity but high solvent concentration by using a cyclic compound such as N-methyl-2-pyrrolidone and γ-butyrolactone as a water-soluble solvent for use in a treatment liquid. However, these solvents have a safety problem, and therefore a final product using these solvents is restricted at the time of import or use.
There are not so many materials effective for solving the problem that a coating of a treatment liquid becomes difficult after the treatment liquid is dried by being left to stand over a long period.